UL synchronization is a procedure that controls the transmit time of a wireless transmit/receive unit (WTRU) in a cell such that UL transmissions from a plurality of WTRUs arrive at a Node-B at the same time. In general, each WTRU has a different propagation delay in a cell covered by a Node-B. Therefore, if there is no synchronization mechanism involved, UL transmissions from WTRUs arrive at different times at the Node-B, which increases intra-cell interference. When UL transmissions are aligned with each other, due to the orthogonality of spreading codes, the intra-cell interference is reduced significantly.
When a WTRU is powered on, the WTRU first establishes downlink (DL) synchronization with a cell using a DL synchronization (SYNC_DL) sequence transmitted in a DL pilot channel (DwPCH). Only after the WTRU has established DL synchronization, can the WTRU start the UL synchronization procedure. UL synchronization is achieved during a random access procedure and therefore involves a UL pilot channel (UpPCH) and a physical random access channel (PRACH).
Although a WTRU establishes DL synchronization and can receive downlink signals from the Node-B, the distance between the WTRU and the Node-B is still uncertain. This leads to unsynchronized UL transmissions. Therefore, the first UL transmission from the WTRU is limited to a special time-slot, an uplink pilot time slot (UpPTS), in order to reduce interference in traffic time slots.
In order to initiate a call, a WTRU first transmits a UL synchronization (SYNC_UL) sequence to a Node-B through a UpPCH. The available SYNC-UL sequences are broadcast through DwPCH. Under the current third generation partnership project (3GPP) standards, eight (8) SYNC-UL sequences are available to a Node-B. For initial transmission of the SYNC_UL sequence, open loop UL synchronization control is used for the UpPCH. The WTRU estimates the propagation delay Δtp based upon the path loss measured on the received primary common control physical channel (P-CCPCH) and/or DwPCH. However, this estimation of the propagation delay is not accurate or reliable.
After the detection of the SYNC-UL sequence in a searching window, the Node-B evaluates the timing of UL transmissions of SYNC_UL sequences, and replies by sending adjustment information to the WTRU to modify its UL transmission timing for the next transmission. This is done with a fast physical access channel (FPACH) within the following four (4) sub-frames. After sending the FPACH, the UL synchronization is established. The UL synchronization procedure is also used for the re-establishment of the UL synchronization when the UL is out of synchronization.